The present invention relates to safety relief valves and to adjustable set pressure safety relief valves in particular.
Safety relief valves are commonly used on pressure vessels and pipelines to relieve temporary pressures in excess of the safe pressure the vessel or pipeline can withstand. Most often a relief valve is built with a range of relief pressures to which it may be set. The advantage of such adjustable safety relief valves is that identical valves may be used for different situations calling for different set pressures. Also, the adjustability makes it unnecessary to manufacture relief valves to critical tolerance if the set pressure can be accurately adjusted once the valve is in place and the system is being tested.
All adjustable safety relief valves have several common elements. All have a body, or housing, an inlet and an outlet, a valve seat, a valve member, and a method of supplying an adjustable force to the valve member to prevent fluids from escaping past the valve seat. Most typically the method of adjusting the force is to rotate an adjustment screw, which in turn changes the compression of a spring member.
The desirable qualities of an adjustable safety relief valve include:
(a) a crisp opening and closing action, PA1 (b) a high flow coefficient, PA1 (c) a low blow down, PA1 (d) a high resolution of the set pressure (ease of adjustability), PA1 (e) reliability, PA1 (f) durability, and PA1 (g) a low cost.
A safety relief valve with a crisp opening action is one which goes from a closed position to a full open position as soon as the set pressure is reached, rather than simmering and slowly opening wider as the pressure further increases. U.S. Pat. No. 4,446,886 to Taylor et al. identifies a method of achieving a crisp opening action by using a huddling chamber: a valve seat and valve member with an increased effective cross-sectional area once fluid starts to flow past the valve seat. A crisp opening makes the valve better serve its purpose of relieving pressure once the pressure gets above the safe point. A crisp closing is beneficial for a similar reason. Once the pressure is reduced, the valve should close rather than allowing more pressure (and the associated fluid) to escape.
The flow coefficient is the ratio of flow through a valve in an open position to flow of the same material and at the same pressure through an orifice the same size as the inlet to the valve. A high flow coefficient means that once the valve opens, there is very little impediment to the escape of fluid and the reduction of pressure. If the flow coefficient is small, a larger valve is needed to make sure that temporary pressure increases can be relieved fast enough to maintain pressures in a safe zone.
The blow down of a valve is the difference between the set pressure and the pressure at which the valve closes. For example, if a valve is set to open at 100 psi, but remains open until the pressure drops to 80 psi, it has a 20% blowdown. While some blowdown is necessary so that the valve does not repeatedly open and close with a pressure near the set point, a large blow down means that the pressure is being reduced much lower than is necessary, with the associated loss of fluid.
The remaining three factors, reliability, durability and cost, relate to practical aspects of valve usage. It is preferable that a valve be reliable and durable, lasting for as long as possible, with a minimum cost. Mechanical simplicity often means a lowr initial cost and less to go wrong. Durability is a function of the material used in construction, but also of the design.
There are several major types of adjustable safety relief valves, each with its own advantages and disadvantages.
Larger sized safety relief valves are usually flanged and have a disk shaped valve member. Illustrative examples of a flanged-disk type valves are depicted in U.S. Pat. Nos. 2,517,858 and 2,821,208 to Farris. In this type of valve, the valve housing is divided by a guide member which holds the valve member in alignment with the valve seat. A stem attached to the valve member extends through the guide member into a compartment containing a spring. On top of the spring is a compression member which is forced down by turning an adjustment screw threaded in the valve housing. Such valves are very high in cost, but have very high flow coefficients (0.975) and good blow down factors (5-7%). They typically have poor opening characteristics and only fair durability, though they have fairly good set pressure resolution.
Set pressure resolution is primarily a function of the spring constant of the spring used. Springs that are weaker (or less stiff) have less of a set pressure change for a given amount of extra compression, which is generally equivalent to a given number of turns of the adjustment screw. Thus, a greater number of turns are required to change the pressure a given amount, and a finer accuracy can be obtained. One problem with using long, weak springs is that, unless supported and guided, they may buckle while being compressed. As a result, some sort of spring guiding is necessary to avoid erratic value action. Outside diameter guiding is common, using a separate guide member surrounding the outside of the annular area occupied by the spring to prevent spring bucking.
Smaller relief valves generally have a threaded body and are of two predominant types: a piston/sleeve type and a ball type. An example of a piston/sleeve type valve is depicted in U.S. Pat. No. 3,572,372 to Moore. In this type of valve the valve member is aligned with the valve seat using a sleeve which extends below the valve seat. When the valve member lifts to let pressure escape, fluid flows out of the sleeve through holes in the walls of the sleeve. The bottom part of the sleeve stays around the inlet forming member. The pressure on the valve member is adjusted by turning an adjustment screw, which is threaded in the top of the valve housing. Riding up and down with the adjustment screw is a compression member on top of the spring. This type of valve is generally fairly high in cost, has good flow coefficients (0.93-0.95), has fair to poor blowdown characteristics (10-25%), poor set pressure resolution, and fair to poor durability. A major problem with the piston/sleeve type of valve is that undesirable material may be present in the escaping fluid which frequently adheres to the cylindrical sliding surfaces preventing proper lifting action of the valve. As a result, this type of valve is totally unsuitable for fluids with contamination which may effect the slidability of the sleeve. In addition, any water or other fluids present around the sleeve may freeze and cause the valve to stick.
Examples of a ball type valve are depicted in U.S. Pat. No. 2,676,782 to Bostock et al. and U.S. Pat. No. 4,446,886 to Taylor et al. In these patents, the valve members are actually spherical. This shape makes it easy to keep the valve members aligned with the valve seats, the inside walls of the valve body generally holding the spheres in the approximate proper position. In both these examples, the spring tension is adjusted by turning an adjustment screw threaded in the top of the valve body, which in turn acts against a compression member. Ball type valves are generally characterized by low cost but very poor blowdown factors (20-50%) and flow coefficients (0.3-0.4). While many have a crisp popping action and good durability, they have only fair set pressure resolution.
There are many other designs which are similar to the ball valve in that the valve member is guided by the inside diameter of the valve body. This type of guiding is susceptible to the sticking problems of the sleeve type valve.
Beside some feature to align the valve member with the valve seat, most valves include some method of limiting the "lift" of the valve member when the valve opens. If the lift is limited, the mechanism aligning the valve member with the valve seat can be shorter. Without a lifting limiting feature, the aligning mechanism will have to extend far enough to match the maximum lift of the valve member.
While individual valve designs each have their particular advantage, no design has heretofore been known which makes it possible to incorporate all the various advantages in one valve. For example, crisp opening valves generally have a high blowdown. If a longer, weaker spring is desired to improve set pressure resolution, the valve body length must be increased at a significant cost. The low cost construction due to simple valve member guiding in ball valves results in poor flow coefficients. Large flanged disk type valves are overly complex and expensive to be reduced in size for smaller applications.